1. Field of the Invention
This application is a continuation-in-part of copending international application Serial No. PCT/US93/11684, filed Dec. 2, 1993, which international application designates the United States.
The present invention relates to the fabrication of multilayer combined rigid and flex printed circuits having. flexible printed circuits extending from the rigid board. In particular, the present invention relates to an improved process for the fabrication of multilayer combined rigid and flex printed circuits, wherein two circuit boards having rigid and flexible sections can be prepared from a novel basestock composite.
2. Background of the Invention
Techniques for making multilayer rigid flex printed circuit boards are well known in the art. One early example of the prior art is disclosed in U.S. Pat. No. 3,409,732, assigned to the Assignee of the present application and whose teachings are incorporated by reference. Another example is disclosed in the parent of instant application, Serial No. PCT/US9311684, which is also assigned to the assignee of this application and whose teachings are incorporated herein by reference. Typically a rigid flex stacked printed circuit board includes flexible printed circuit cables extending from the periphery of the rigid section or sections. The rigid portions of the flex cables are typically used as sites for electronic components or mechanical hardware. It is important to note that the copper conductor in each plane or layer is fabricated from one continuous sheet of copper foil.
With improvements in electronic technology, there has been a constant need for advances in electronic packaging. This need has led to more complex multilayer rigid flex printed circuit boards with many boards now using up to twenty-five, or even more, layers of circuitry. However, severe problems developed when the rigid circuit portions included many layers of conductors and holes plated through with copper to provide conductor barrels connecting the conductor layers.
One particular problem, reported on and discussed in U.S. Pat. No. 4,800,461, assigned to the assignee of the present Application, and whose teachings are incorporated by reference, described the fact that in multilayer rigid flex boards which include insulator materials such as acrylic adhesive and Kapton (Kapton is a trademark of E. I. dupont de Nemours and Company Inc. for polyimide film), the insulating materials placed a "Z-axis" stress on plated through holes. The coefficient of thermal expansion, it was reported, of the acrylic adhesive (Z-axis expansion) was the dominate influence. It was observed that because of the amount of acrylic required in many multilayer rigid flex applications, all plated through holes are stressed, with many cracking, making the boards unusable.
To overcome this problem, the '461 patent reported on a novel process to provide a rigid section incorporating insulator materials which, when subjected to elevated temperatures, did not expand in the Z direction to cause difficulties, including delamination and cracking of plated copper barrels. Stated another way, in the '461 patent, the materials causing undesirable expansion in the Z direction in the multilayer rigid section of the board, and the materials absorbing excessive amounts of moisture, such as acrylic adhesives and Kapton, were eliminated from the boards rigid section.
However, although the '461 patent was extremely successful in addressing the various problems recited therein, and in particular, the problem of thermal stresses described above, the process for fabrication of the rigid flex printed circuits has remained limited to the fabrication of a multilayer combined rigid and flex printed circuit board wherein two circuit boards are always prepared from a basestock composition, and remain attached to one another via the prepreg. In other words, the process of fabrication according to the teachings of the prior art begin with the step of laminating two conductor layers (i.e. copper layers) to a single insulator layer (prepreg) followed by imaging and etching. Accordingly if one of the two bonded conductor layers was somehow improperly imaged, it was necessary to discard the entire lamination.
Other problems also persist. One problem concerns the inability to satisfactorily mount die and interconnection lines to flexible substrate sections. Substrates currently in use distort and/or frequently delaminate when subjected to thermosonic bond techniques used to bond die, and during formation of fine line interconnection lines in the substrate. These conditions can yield unsatisfactory results in applications where high levels of design integration and uniformity among similar features on the substrate are required.
It is therefore an object of this invention to provide a novel basestock composition of sufficient planarity and rigidity to permit adequate handling and further processing thereof, and which can be separated into two individual layers each comprising an imaged copper layer laminated to a fiberglass sheet which is impregnated with adhesives such as an epoxy, which has been laminated and bonded to acrylic coated polyimide film.
It is yet another object of this invention to provide a multilayer combined rigid and flex printed circuit substrate that provides sufficient rigidity to permit satisfactory mounting of die using thermosonic bonding techniques and formation of fine line interconnections.